Well, if you've got just one or two small newts in an almost full 10 gallon with a filter and live plants and it's cycled, I'd say every other week. You should get a test kit. Once your tank is cycled, you'll have 0 ammonia, 0 nitrites, and then you'll have to test for nitrates, and when they get high, over 20ppm or so, then do a water change.